(a) Field of the Invention
The present invention relates to a clean steel made of a ferroalloy of super high purity, and relates to a clean steel containing extremely small amounts of oxygen, sulfur and nitrogen, and small amounts of magnesium and calcium.
(b) Related Art Statement
The inventor has previously proposed a method of manufacturing molten steel having less contents of oxygen and sulfur as Japanese Patent Laid-open No. 52(1977)-58,010 and Japanese Patent Application Publication No. 62(1987)-37,687.
The inventor has further proposed an iron-, nickel-, and cobalt-base alloy having extremely small contents of sulfur, oxygen and nitrogen and a method of manufacturing the same as Japanese Patent Laid-open No. 62(1987)-83,435.
According to the above prior methods, the residual sulfur is less than 0.002%, the residual oxygen is less than 0.002% and the residual nitrogen is less than 0.03% in the molten steel.
That is, the invention of Japanese Patent Laid-open No. 62(1987)-83,435 relates to a method of manufacturing an iron-base alloy having extremely small contents of oxygen, sulfur and nitrogen comprising a step of substantially melting an iron alloy in a crucible consisting of basic refractories containing 15-75 wt % of MgO and 15-85 wt % of CaO, or a crucible, a crucible melting furnace, a converter or a vessel such as a ladle lined with said refractories, deoxidizing, desulfurizing and denitrifying the molten alloy in a non-oxidizing atmosphere such as argon gas, nitrogen gas or helium gas or in vacuo, by adding first and second additives, the first additive being aluminum or an aluminum alloy, and the second additive being selected from the group consisting of boron, alkali metal and alkali earth metal, and casting the thus deoxidized, desulfurized and denitrified molten alloy into an ingot.
According to this method, in order to remain
______________________________________ residual Al 0.005-7% residual Mg 0.005-0.0001% residual Ca 0.005-0.0001% total residual amount of at least 0.001-10 wt % one element selected from the group consisting of boron, alkali metal and alkali earth metal ______________________________________
these metals are preferably added.